In parts and housings of building materials, electrical and electronic appliances (e.g., personal computers, office automation equipment, audiovisual equipment and cellular phones), optical instruments, precision instruments, toys, household/office electric appliances and the like, particularly in parts and molded materials that are utilized in the fields of transit and transportation industries, such as railway vehicles, automobiles, ships and airplanes, vibration damping properties are demanded in addition to general material properties, such as impact resistance, heat resistance, strength and dimensional stability. Therefore, vibration dampers are used for such parts and molded materials.
The vibration dampers include low-hardness products having gel-like properties, which are used for soles of athletic shoes, etc., and high-hardness products having relatively high hardness, which are used for support materials of audio equipment, etc.
In the case of the low-hardness products, various low-rebound resilience materials are produced by processing them into gels or sponges, and they are widely used. Also in the case of the high-hardness products, low rebound resilience is demanded. In the conventional high-hardness products, however, if high hardness is maintained, low rebound resilience is not obtained, and if low rebound resilience is promoted, high hardness is impaired, so that there is a problem of difficulty in making high hardness and low rebound resilience compatible with each other.
As a high-hardness vibration damper, a vibration damper comprising an olefin-based polymer constituted of 50 to 100% by weight of structural units derived from 4-methyl-1-pentene and 0 to 50% by weight of structural units derived from at least one olefin selected from olefins having 2 to 20 carbon atoms, except 4-methyl-1-pentene, is disclosed in, for example, a patent literature 1.
In a patent literature 2, a low-rebound resilience/vibration damping polymer composition in which into a copolymer such as polyurethane an active component to increase the amount of dipole moment of the copolymer has been compounded is disclosed.
In a patent literature 3, a shock absorber composition comprising a copolymer containing a vinyl aromatic compound, wherein a tan δ peak obtained by dynamic viscoelasticity measurement (1 Hz) of the copolymer is in the range of higher than 0° C. but not higher than 20° C., a tan δ value is not less than 0.4 in the total temperature range of 5° C. to 15° C., and a tan δ value at 15° C. is not less than 0.5, is disclosed.
In a patent literature 4, a rubber composition having at least one peak of loss tangent (tan δ) in the range of −60 to −30° C., the loss tangent being measured at 100 rad/sec, and at least one peak thereof in the range of 0 to 40° C., wherein the rubber composition comprises an acrylic copolymer in which an α,β-unsaturated nitrile monomer has been copolymerized, and an ethylene/α-olefin-based copolymer having a peak of the above tan δ in the range of −60 to −30° C., is disclosed.
In any of the above techniques, however, compatibility of high hardness and low rebound resilience with each other, particularly compatibility of high hardness such as a durometer hardness of 50 to 80 and low rebound resilience such as a rebound resilience of not more than 20% with each other has not been sufficiently realized.